Three words you're unlikely to hear from a UK or US politician these days are "war on terror", but undeniably the phrase has taken its place just behind "yes we can" and "I agree with Nick" in the roll-call of classic political soundbites. The latest instalment of this saga played out on Thursday, when an al-Qaida cell was apprehended in Norway, and some of the details being reported paint a depressingly familiar picture to anyone who follows this sort of thing.

The makeup of the Norwegian terrorist cell – like the failed New York subway bombers and the 7/7 bombers – is notable for its complete and total remove from Afghanistan. It comprises a Norwegian national of Uighur origin, and an Uzbek citizen and an Iraqi who both have indefinite leave to remain in Norway. A fourth suspect was arrested in Germany. Their intended targets are as yet unclear, and speculation as to their motives has inevitably turned to Norway's 500 troops in Afghanistan and the decision by some Norwegian papers to republish the Danish cartoons of the Prophet – a commendable stand for press freedom, which set an example sadly not followed by any UK publication.

When al-Qaida burst into the foreground of western consciousness in September 2001, there was a scramble to understand what sort of entity we were dealing with. The surface picture quickly fell into place – they were a terrorist organisation centred around Osama bin Laden, with infrastructure and training camps in Taliban-controlled Afghanistan. The response seemed clear cut – remove al-Qaida from Afghanistan, capture Bin Laden and topple his Taliban facilitators so that al-Qaida can't take root there again. There was little dissent from the notion that this course of action would make western streets safer.

Fast-forward nine years, and we all know that the situation was always much more complex. Well, almost all of us – just last week, defence secretary Liam Fox declared that our engagement in Afghanistan is "a national security imperative", wheeling out once again the time-worn argument that military actions in Afghanistan can reduce the terrorist threat in the west. This issue is proving to be a gift-wrapped package for any politician in opposition – Bob Ainsworth had to hold the same line in the face of mounting Tory derision when he was defence secretary for Labour, and now it's Labour's turn to point and laugh at Fox when he's forced to do the same.

I can understand why Ainsworth felt the need to cleave to the idea that our domestic security depends on a stable Afghanistan, since it was his party leaders that committed us to the invasion. Any admission of doubt would have been politically disastrous for Labour. To a lesser degree, I can understand why Fox cleaves to it too – in for a penny, and all that. But it seems that there's only so far an argument can stand in complete opposition to an increasingly obvious reality, and this one is now stretched to breaking point.

Since 2001, a whole shelf of books have been published about al-Qaida, and the overall consensus is that they are now no more than a brand, a nebulous idea that anyone can subscribe to. Many existing Islamist groups simply changed their name to "al-Qaida", without necessarily establishing any direct contact with Bin Laden or Ayman al-Zawahiri.

Even more impossible to counter militarily are al-Qaida cells such as the 7/7 bombers, who showed that it only takes seven or eight disaffected young men, a broadband connection and some household items (like peroxide and flour) to create an al-Qaida cell. That's not to say that we haven't been successful in uncovering these operations – the Norway episode is the latest in many successes on this front – but it's impossible to see how our engagement in Afghanistan has helped this effort.

So how can anyone still honestly maintain that our involvement in Afghanistan can reduce the terrorist threat level on our streets? Practically every al-Qaida convert in the last nine years has cited the invasion as a galvanising factor in their radicalisation, so in fact our involvement seems to have had the opposite effect. And that's leaving aside the hard-earned lesson that the border between the uncontrollable region of north-west Pakistan and Afghanistan exists largely in our imaginations.

There is a separate argument to be made that a stable, democratic Afghanistan is in the world's best long-term interest, provided you don't think too much about our chances of achieving that goal within the available timeframe. So why is Fox not focusing on that argument alone? Why has the "national security" justification not joined the phrase "war on terror" on the scrapheap of bad ideas? Fox could (just about) afford to do that, whereas Ainsworth couldn't. In one fell swoop he'd not only insulate himself from easy blows from the opposition, but he'd also reassure the UK public that government ministers can take account of widely acknowledged realities when they formulate their arguments.

Tony Blair's foreign policy decisions are proving to be like the incomprehensible derivatives that helped bring down the financial system in 2008 – ticking time-bombs that don't reveal their true, noxious nature until it's too late. This particular one can now be seen for what it is, which is why we need to cut it loose.